So, I will be running a game soon. I have a initial overall plot outline, but I am having trouble with how to get the plot started. Basically, i have an idea of where i want the players to go, in this case the Underdark. What i am having trouble with is coming up with a compelling but believable reason that their characters specifically should be the ones to go there, and what the purpose is. I want this to be the main MacGuffin of the entire campaign. I have lots of ideas of things to happen along the way, just trouble coming up with why it is they would make a long adventure to the Underdark.
And the bride/groom wasn't actually kidnapped . . . they ran away to be with their svirfneblin lover! But one of the star-crossed perishes, and the other swears revenge on the (surface world/Underdark) for their cruelty!
Motivation works best when it comes from the characters. For example, it's useless to have a MacGuffin that will save the world if your characters are all self centered or evil. They won't care.
I have a side plot in my campaign where the mentor to one of the PCs was captured and held as a slave by the drow ruling house in Zyrnakaynen. It's up to the PCs to make the effort to go there to rescue her when they are ready. If it was the main plot, I would have to make that mentor one that is close to them all in order to properly motivate them.